This invention relates generally to signal processing in synthetic aperture radar (SAR) systems. This invention relates particularly to a method for compensating for motion of an aircraft carrying an SAR system. Still more particularly, this invention relates to a method for using global positioning system (GPS) data with an inertial navigation system, (INS) to provide smooth and accurate host vehicle motion compensation.
SAR radar system is described in Stimson "Introduction to Airborne Radar", 1983, Hughes Aircraft Company, pp. 527-580. Integrated INS/GPS systems are described in a paper presented by Lipman at the Institute of Navigation GPS-92 Technical Meeting, Sep. 16-18, 1992 entitled "Tradeoffs in the Implementation of Integrated GPS Inertial Systems". Additional descriptions of integrated INS/GPS systems are described in a paper by Lipman et al. at the Precision Strike Technology Symposium, Johns Hopkins University Applied Physics Laboratory, Sept. 27-28, 1995, entitled "Tradeoffs and Technical Issues in the Integration of Modern Navigation Systems and Synthetic Aperture Radars"; and in a paper by Lipman at the Precision Strike Technology Symposium, Johns Hopkins University Applied Physics Laboratory Oct. 9-10, 1996, entitled "Millimeter Navigation Performance for High Quality SAR Imaging and Target Location." The above-cited papers by Lipman and Lipman et al. are incorporated by reference into the present disclosure as be being background material and an indication of the state of the art in motion compensation in SAR systems.
The image quality of synthetic aperture radar depends on the accurate knowledge of the host vehicle position and the smoothness of the relative change in position error. The use of an integrated INS/GPS system to provide this motion compensation has become an important source of this information.
The use of integrated ring laser gyro inertial system with embedded global positioning system receivers for motion compensation support of synthetic array radar imaging has provided an important advance in picture quality and ability to accurately locate objects. A universal dilemma that confronts high-accuracy SAR applications is how to maintain the increased accuracy that the Kalman filtered GPS data provides the inertial system and minimize the disturbing effects of the steps or discontinuity in inertial data due to Kalman updates.
With the development of high resolution SAR that provides image gathering over longer periods of time and at longer ranges and different frequencies, the need for even higher levels of performance than normally available from an integrated GPS/INS system is now desired. The key navigation requirement for accurate SAR target designation is two-fold: (1) accurate absolute velocity and position, and (2) very smooth low relative position error noise over the image gathering period.
In typical aircraft applications of GPS/INS systems the basic accuracy of the blended (or hybrid) solution is provided by the GPS navigation solution. The INS provides the short term navigation between updates and serves as an aid which raises the navigation system tolerance to jamming and loss of satellite signal. The military aircraft systems are implemented with a Kalman filter and mechanization that was designed to be robust under dynamic conditions and provide updates to maximize system accuracy whenever additional data was received from the satellite. Under integrated GPS/INS operation using precision performance service, navigational performance on the order 5 to 10 meter circular error probable (CEP) and time RMS velocity accuracy of the order of 0.025 meter/second is commonly provided. While this performance and architecture satisfy some of the needs of the SAR targeting operation, it may not always lend itself to other aspects. In particular the quality of the SAR image is a critical function of the noise characteristic in the position data and requires very accurate relative position change during the SAR scene time.
The optimum real time output provided by an INS/GPS system is based on a Kalman filter mechanization. The use of a Kalman filter in a classical implementation introduces discontinuities at the normal discrete updates of GPS information into the smooth inertial data. To avoid this problem methods used have been ad hoc type implementations where the Kalman updates are leaked or bled in to maintain the best real time accuracy and minimize the unnatural disturbance to the smoothness of the inertial data.
An important point has been missed in these types of mechanizations is that is the SAR image is not a true real time application. It is a "near real time" application. The data gathering period prior to an attempt to form an image can be a period of several seconds to a large fraction of a minute or more. During that period it is necessary to provide motion compensation data which can then be altered by various schemes during the image forming process. These schemes are generally referred to as "autofocus techniques" but do not depend on navigation velocity and position accuracy improvement but on other image processing related characteristics.